Preheat your oven to 350°F (175°C).
In a bowl, mix together flour, baking powder, and salt. Set aside.
In a separate bowl, cream the softened butter and sugar until light and fluffy.
Add the egg and peppermint extract to the butter mixture, beating until combined.
Gradually add the dry ingredients to the wet mixture, mixing until just combined.
Divide the dough in half. Add red food coloring to one half and green to the other, blending thoroughly.
Scoop tablespoon-sized balls from each dough color and drop them onto a baking sheet, alternating colors for a swirled effect.
Sprinkle crushed peppermint candies on top of the cookie mounds before baking.
Bake for 10-12 minutes or until the edges are lightly golden.
Allow the cookies to cool on the baking sheets for 5 minutes, then transfer them to wire racks to cool completely.